Gaskets are required to have properties including airtightness and low sliding resistance. Airtightness refers to the ability to allow the liquid contents to be used without leaking outside and also prevent outside foreign substances from entering. Low sliding resistance refers to the ability to allow the operator of the prefilled syringe to move the gasket using the plunger with one hand without difficulty.
Higher airtightness between the gasket and the inner periphery of the barrel increases the sliding resistance of the gasket to the inner periphery, thereby deteriorating operability. For example, a method for solving this problem is known which improves the sliding properties of the outer surface of a gasket by laminating the outer surface with the low-friction material fluororesin (Patent Literature 1). However, when the gasket laminated with such a fluororesin film is inserted into a barrel, the maximum diameter portion (peak portion) of the gasket contracts due to a reaction force from the inner face of the barrel, thereby causing wrinkles or sagging in the fluororesin film on the portion. The wrinkles form gaps between the peak portion of the gasket and the inner face of the barrel, thereby reducing the airtightness of the gasket so that liquid leakage can be caused.